In recent years, extensive research has been conducted on the separation and purification of a fluid mixture using a selectively permeable membrane in place of conventional techniques based on a phase change such as distillation and freezing, which require a large amount of energy.
Processes for separation and purification using such a membrane which are presently in practical use on a commercial scale are directed mainly to liquid/liquid separation, such as the production of fresh water from sea water, disposal of waste water from factories, and concentration of foods, and liquid/solid separation. In connection with gas/gas separation, basically no process has yet been put to practical use. The major reasons for this are:
(1) selective permeability or gas selectivity is poor; that is, since no membrane is available which allows a specific gas to pass therethrough but does not essentially allow other gases to pass therethrough, it is necessary to employ a multistage system in which membrane separation is repeatedly applied when a specific gas of high purity is to be produced. Accordingly, large-sized equipment is needed; and PA1 (2) gas permeability is poor; therefore, it is difficult to treat a large amount of a gas mixture. In particular, when gas selectivity is increased, gas permeability tends to drop, whereas when gas permeability is increased, gas selectivity tends to fall. This problem has not yet been satisfactorily overcome. PA1 a gas-selectively permeable membrane which is an asymmetrical pore diameter structure film made of a polyetherimide having the recurring unit represented by formula (A) as described hereinafter, or a mixture of the polyetherimide and at least one polymer having the recurring unit represented by formula (B) as described hereinafter, wherein the mean pore diameter of a dense layer of the asymmetrical pore diameter structure film is 0.5 micron or less and the mean thickness of the dense layer is 10 microns or less; and PA1 a composite gas-selectively permeable membrane comprising the asymmetrical pore diameter structure film as described above and at least one thin polymer film provided on the dense layer of the asymmetrical pore diameter structure film; and PA1 a method of forming the gas-selectively permeable membrane as described above which comprises applying a solution containing the polyetherimide having the recurring unit represented by formula (A) as described hereinafter, or a mixture of the polyetherimide and at least one polymer having the recurring unit represented by formula (B) as described hereinafter, a solvent, and if desired or necessary, a swelling agent to form a film, bringing the thus-formed film into contact with a coagulating agent to remove the solvent, and then drying; and PA1 a method of forming the composite gas-selectively permeable membrane as described above which comprises further providing the thin polymer film on the gas-selectively permeable membrane.
Typical membrane forming methods which have been employed to prepare a satisfactory membrane include a method in which an asymmetrical pore diameter structure membrane whose active skin layer is reduced in thickness as much as possible is formed by casting a polymer solution, and a method in which a superthin membrane corresponding to the active skin layer is separately prepared and provided on a porous support to form a composite membrane. These methods to improve gas permeability are not always suitable for practical use because commercially available polymers fail to satisfy all desired physical properties; that is, commercially available polymers or copolymers are satisfactory in at least one of selective permeability, permeability, heat resistance, chemical resistance, strength, and so forth, but are not satisfactory in other properties.
It has, therefore, been desired to develop polymers which are of high heat resistance and can be produced inexpensively. A typical example of such a polymer is a polysulfone. This polysulfone, however, is not satisfactory in respect of production cost.